Environmental Engineering Reference
In-Depth Information
3.3.4
L
IQUID
-L
IQUID
E
QUILIBRIUM
3.3.4.1
Octanol-Water Partition Constant
If two liquid phases are in contact, and a solute is present in both, then at equi-
librium we have a distribution of solute between the two phases consistent with
equal chemical potentials or fugacity values. In environmental engineering, liquid-
liquid equilibrium is common: for example, the distribution of organic chemicals in
the water environment where a third phase (e.g., oil) is present, such as occurring
during oil spills at sea and inland waterways, floating oils in wastewater treatment
plants, and sub-surface spills in contact with groundwater. Solvent extraction is a
well-known operation in environmental engineering separation processes. Since the
concept of like dissolves like is mostly true, it should be expected that most organic
compounds would have a greater affinity for organic solvents and substrates. A spe-
cific liquid-liquid system (octanol-water) has special relevance to environmental
engineering.
Consider a solute
i
that is distributed between two solvents (octanol
≡
o and water
≡
w). At equilibrium the solute
i
should have equal fugacity in both phases. Thus
f
i
f
i
,
=
(3.47)
the total number of moles of
i
in the system. We have
x
i
γ
o
i
f
lo
x
i
γ
w
i
f
lo
=
,
(3.48)
i
i
and therefore
x
i
w
i
x
i
=
γ
K
ow
=
(3.49)
o
i
γ
is the
partition constant
for a solute between two phases defined as the ratio of mole
fractions.
A large body of literature exists on the partitioning of a variety of environmen-
tally significant compounds between the organic solvent, 1-octanol, and water (Leo,
Hansch, and Elkins, 1971). The octanol-water partition constant defined in terms of
the ratio of the molar concentrations of solute
i
in both phases is designated
K
ow
:
γ
V
w
V
o
,
x
i
V
w
w
i
γ
C
i
o
C
i
w
=
K
ow
=
x
i
V
o
=
(3.50)
o
i
where
V
w
and
V
o
are the partial molar volumes of water and 1-octanol, respectively.
The availability of such a large database on
K
ow
is not entirely fortuitous. It has
long been a practice in pharmaceutical sciences to seek correlations of the various
properties of a drug with its
K
ow
. 1-Octanol appears to mimic the lipid content of biota
verywell.Asimilarreasoningledtoitsacceptanceasadescriptorofchemicalbehavior
in the environment. 1-Octanol has the same ratio of carbon to oxygen as the lipids
and represents satisfactorily the organic matter content in soils and sediments. It is
Search WWH ::
Custom Search